Why use present tense in the following sentence
my text book says"Ann isn't free on Saturday. She is working"
why not use future tense and say"Ann won't be free on Saturday. She is working"

Question

Answer

You can use present simple tense to state a fact.  By using "won't", you're only making a prediction.

Answer

The tense here is to tell people "Ann is not free for almost every Saturday, not just next Saturday".

Answer

Have a look at: http://www.englishgrammarsecrets.com/presentformsforfuture/menu.php

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